1. Technical Field
The disclosure relates to an exhaust gas control system and an exhaust gas control apparatus for an internal combustion engine.
2. Description of Related Art
Japanese Patent Application Publication No. 2014-88800 (JP 2014-88800 A) discloses an internal combustion engine that is provided with a catalyst which removes nitrogen oxide (NOx) in exhaust gas. An adding mechanism that adds urea water to the exhaust gas is disposed in an exhaust passage of the internal combustion engine, and ammonia resulting from the urea water is adsorbed onto the catalyst for the NOx removal. The NOx is reduced and removed by the use of the ammonia that is adsorbed on the catalyst.
When the amount of the ammonia adsorbed on the catalyst is insufficient, an appropriate NOx removal cannot be performed. When the ammonia adsorption amount is excessive, an ammonia slip is likely to occur. Accordingly, it is desirable to execute an ammonia adsorption amount control, which sets a target adsorption amount for the ammonia adsorbed onto the catalyst and controls a urea water addition amount based on the target adsorption amount.
Inhibiting an error between the target adsorption amount and the actual ammonia adsorption amount is important in executing the ammonia adsorption amount control. When a high-temperature state of the catalyst continues for a long period of time, however, the amount of the ammonia that is desorbed from the catalyst increases, and thus the actual ammonia adsorption amount deviates from the target adsorption amount, which causes error to be accumulated.
In the apparatus that is disclosed in JP 2014-88800 A, for example, the temperature of the catalyst is raised by a processing for raising the temperature of the exhaust gas being executed when a deviation between an estimated value of the ammonia adsorption amount and the actual ammonia adsorption amount becomes equal to or greater than a predetermined value. This allows an initialization processing, by which the ammonia is entirely desorbed from the catalyst, to be performed. When the initialization processing is performed, the accumulated error is eliminated, and thus the deviation of the actual ammonia adsorption amount with respect to the target adsorption amount can be inhibited during the subsequent ammonia adsorption amount control.